This invention relates generally to medical devices, and more particularly to a system and method for discriminating supraventricular tachycardia from ventricular tachycardia during a tachycardia event.
The heart is generally divided into four chambers, the left and right atrial chambers and the left and right ventricular chambers. As the heart beats, the atrial chambers and the ventricular chambers go through a cardiac cycle. The cardiac cycle consists of one complete sequence of contraction and relaxation of the chambers of the heart. The terms systole and diastole are used to describe the contraction and relaxation phases the chambers of the heart experience during a cardiac cycle. In systole, the ventricular muscle cells contract to pump blood through the circulatory system. During diastole, the ventricular muscle cells relax, causing blood from the atrial chamber to fill the ventricular chamber. After the period of diastolic filling, the systolic phase of a new cardiac cycle is initiated.
Through the cardiac cycle, the heart pumps blood through the circulatory system. Effective pumping of the heart depends upon five basic requirements. First, the contractions of cardiac muscle must occur at regular intervals and be synchronized. Second, the valves separating the chambers of the heart must fully open as blood passes through the chambers. Third, the valves must not leak. Fourth, the contraction of the cardiac muscle must be forceful. Fifth, the ventricles must fill adequately during diastole.
When the contractions of the heart are not occurring at regular intervals or are unsynchronized the heart is said to be arrhythmic. During an arrhythmia, the heart""s ability to effectively and efficiently pump blood is compromised. Many different types of arrhythmias have been identified. Arrhythmias can occur in either the atrial chambers or in the ventricular chambers of the heart.
Ventricular fibrillation is an arrhythmia that occurs in the ventricle chambers of the heart. In ventricular fibrillation, various areas of the ventricle are excited and contract asynchronously. During ventricular fibrillation the heart fails to pump blood. Since no blood is pumped during ventricular fibrillation, the situation is fatal unless quickly corrected by cardiac conversion. Ventricular tachycardia is another arrhythmia that occurs in the ventricular chambers of the heart. Ventricular tachycardia is a very serious condition. Ventricular tachycardias are typified by ventricular rates between 120-250 and are caused by disturbances (electrical or mechanical) within the ventricles of the heart. During a ventricular tachycardia, the diastolic filling time is reduced and the ventricular contractions are less synchronized and therefore less effective than normal. Ventricular tachycardias must be treated quickly in order to prevent the tachycardia from degrading into a life threatening ventricular fibrillation.
Arrhythmias that occur in the atrial chambers of the heart are referred to generally as supraventricular tachycardias. Supraventricular tachycardias include atrial tachycardias, atrial flutter and atrial fibrillation. During certain supraventricular tachycardias, aberrant cardiac signals from the atria drive the ventricles at a very rapid rate. Such a situation occurs during paroxysmal atrial tachycardia. This condition begins abruptly, lasts for a few minutes to a few hours, and then, just as abruptly, disappears and the heart rate reverts back to normal.
Cardioverter-defibrillators, such as implantable cardioverter-defibrillators (ICDs), have been shown to be effective in reducing the incidence of sudden cardiac death. Sudden cardiac death is typically caused by either ventricular tachycardia or ventricular fibrillation. Cardioverter-defibrillator systems operate by sensing and analyzing cardiac signals and applying electrical energy to the heart when either a ventricular tachycardia or ventricular fibrillation is detected. One common way cardioverter-defibrillators detect cardiac arrhythmias is to sense and analyze the rate of ventricular contractions. When the ventricular rate exceeds a programmed threshold value, the cardioverter-defibrillator applies electrical energy in one or more specific patterns to treat either the ventricular tachycardia or ventricular fibrillation.
Rapid ventricular rhythms, however, can occur in the presence of a supraventricular tachycardia. As previously mentioned, one example is during paroxysmal atrial tachycardia. In this situation, treating the ventricles with electrical energy is inappropriate as the treatment does not address the precipitating factor of the rapid ventricular rate. Therefore, a need exists for reliably assessing and determining the origin of a rapid ventricular rate. By reliably discriminating the origin of the rapid ventricular rate, more appropriate and effective therapies can be applied to treat the heart.
The present subject matter discloses a method and a system for discriminating, or classifying supraventricular tachycardias (SVT) from malignant ventricular tachycardias (VT) during a tachycardia event. In one embodiment, the present subject matter is implemented in an implantable cardioverter defibrillator. By using the method of the present subject matter, the implantable defibrillator assesses and determines the origin of a rapid ventricular rate, allowing the implantable defibrillator to reduce the number of inappropriate therapies delivered to the heart.
In one embodiment, QRS-complexes are sensed during normal sinus rhythm (NSR). A plurality of feature points are located on the sensed NSR QRS-complexes based on morphological features of the individual NSR QRS-complexes. The plurality of feature points from the NSR QRS-complexes are then used to determine a NSR template. In one embodiment, a plurality of NSR QRS-complexes are used to determine the NSR template.
In one embodiment, the NSR template includes a median value for each of the plurality of feature points taken along the NSR QRS-complex. A numerical convolution is then preformed on the values of the NSR template. A numerical convolution is also preformed on the plurality of feature points for each of the plurality of the NSR QRS-complexes. This process gives a NSR filter output for each of the NSR QRS-complexes. Using the NSR filter output for each NSR QRS-complex, a median NSR filter output template is determined. In one embodiment, the median NSR filter output template includes the median values of the NSR filter output values for each NSR QRS-complex.
Once the median NSR filter output template has been determined, the system senses for the occurrence of a tachycardia event. When a tachycardia event is detected, the system senses the tachycardia complexes. In one embodiment, QRS-complexes are extracted, or sampled, from the tachycardia complexes in the sensed signals. The plurality of feature points are then located in the QRS-complexes. The feature points located in the QRS-complexes during the tachycardia event are based on morphological features of the QRS-complex. In one embodiment, the morphological features taken from the QRS-complex during the tachycardia episode are from the same relative position as the morphological features taken along the NSR QRS-complex.
A tachycardia complex output is then determined by performing a numerical convolution of the median NSR filter output template with the plurality of feature points from a QRS-complex of a tachycardia complex sensed during the tachycardia event. The differences between the values of the tachycardia complex output and the median NSR filter output template are summed to give a sum of residual value. The sum of residual (SOR) value is then compared to a sum of residual (SOR) threshold value, and when the SOR value is greater than or equal to the SOR threshold value the tachycardia complex is classified as a ventricular tachycardia complex. When the SOR value is less than the SOR threshold value the tachycardia complex is classified as a supraventricular tachycardia complex. When the number of tachycardia complexes classified as either ventricular tachycardia complexes or supraventricular tachycardia complexes exceeds a predetermined threshold value, the tachycardia event is classified as the tachycardia event that exceeded the threshold value (i.e., as either a ventricular tachycardia or a supraventricular tachycardia).
These and other features and advantages of the invention will become apparent from the following description of the preferred embodiments of the invention.